What he does for the team...there's very few in the world who could do it better. You're talking literally the best players going IMO. Messi, Sterling, De Bruyne, Mane, that sort of silly level.
Take these brilliant players, just in the PL. Maddison, Son, Zaha, Salah, Bernardo Silva, Mahrez, Wijnaldum, Pogba. On current form, would any of them do better for us than Jack right now? Contribute everything he does offensively and continue to run themselves into the ground defending when surrounded by a team nowhere near as good? How do I know they wouldn't crumble under pressure when our team has one of their funny spells when they all forget how to play football?
I wouldn't swap him for anyone honestly, I enjoy watching him too much. Plus, if you could magically switch Grealish for one of the those top players, it would take time for them to adjust to their new teammates much worse than wherever they've come from.